In the well-known process of electrophotographic printing, such as in xerography or ionography, a charge retentive surface, typically known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder known as "toner." Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is useful for light lens copying from an original or printing electronically generated or stored originals such as with a raster output scanner (ROS), where a charged surface may be imagewise discharged in a variety of ways.
In the process of electrophotographic printing, the step of conveying toner to the latent image on the photoreceptor is known as "development." The object of effective development of a latent image on the photoreceptor is to convey toner particles to the latent image at a controlled rate so that the toner particles effectively adhere electrostatically to the charged areas on the latent image. A commonly used technique for development is the use of a two-component developer material, which comprises, in addition to the toner particles which are intended to adhere to the photoreceptor, a quantity of magnetic carrier beads. The toner particles adhere triboelectrically to the relatively large carrier beads, which are typically made of steel. When the developer material is placed in a magnetic field, the carrier beads with the toner particles thereon form what is known as a magnetic brush, wherein the carrier beads form relatively long chains which resemble the fibers of a brush. This magnetic brush is typically created by means of a "developer roll." The developer roll is typically in the form of a cylindrical sleeve rotating around a fixed assembly of permanent magnets. The carrier beads form chains extending from the surface of the developer roll, and the toner particles are electrostatically attracted to the chains of carrier beads. When the magnetic brush is introduced into a development zone adjacent the electrostatic latent image on a photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be pulled off the carrier beads and onto the photoreceptor. Another known development technique involves a single-component developer, that is, a developer which consists entirely of toner. In a common type of single-component system, each toner particle has both an electrostatic charge (to enable the particles to adhere to the photoreceptor) and magnetic properties (to allow the particles to be magnetically conveyed to the photoreceptor). Instead of using magnetic carrier beads to form a magnetic brush, the magnetized toner particles are caused to adhere directly to a developer roll. In the development zone adjacent the electrostatic latent image on a photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be pulled off the developer roll and onto the photoreceptor.
An important variation to the general principle of development is the concept of "scavengeless" development. The purpose and function of scavengeless development are described fully in, for example, U.S. Pat. No. 4,868,600 to Hays et al., U.S. Pat. No. 4,984,019 to Folkins, or U.S. Pat. No. 5,010,367 to Hays. In a scavengeless development system, toner is conveyed to the photoreceptor by means of AC electric fields supplied by self-spaced electrode structures, commonly in the form of wires extending across the photoreceptor, positioned within the nip between a donor roll and photoreceptor. Because there is no physical contact between the development apparatus and the photoreceptor, scavengeless development is useful for devices in which different types of toner are supplied onto the same photoreceptor, as in "tri-level" or "recharge, expose, and develop" highlight or image-on-image color xerography.
A typical scavengeless development apparatus includes, within a developer housing, a magnetic roll, a donor roll, and an electrode structure. The magnetic roll operates in a manner similar to a developer roll, but instead of conveying toner directly to the photoreceptor, conveys toner to a donor roll disposed between the magnetic roll and the photoreceptor. The magnetic roll is electrically biased relative to the donor roll, so that the toner particles are attracted from the magnetic roll to the donor roll. The donor roll further conveys toner particles from the magnetic roll toward the photoreceptor. In the nip between the donor roll and the photoreceptor are the wires forming the electrode structure. During development of the latent image on the photoreceptor, the electrode wires are AC-biased relative to the donor roll to detach toner therefrom so as to form a toner powder cloud in the gap between the donor roll and the photoreceptor. The latent image on the photoreceptor attracts toner particles from the powder cloud, forming a toner powder image thereon.
No matter what specific type of development is used, a primary factor affecting copy quality is the purity of the toner. There are two common sources of toner defects which are likely to cause copy quality problems, particularly in scavengeless systems: agglomeration and contamination. Agglomeration is the "clumping" of loose toner particles in the developer housing, and is often the result of high temperatures or mechanical abrasion. Large clumps of agglomerated toner may adhere to the magnetic brush or developer roll and cause streaking against the photoreceptor, which may become apparent as a copy quality defect for numerous copies or prints. Another key source of defects is external contamination of the toner by foreign objects. Common sources of toner contamination include solid shavings created in the manufacture of the apparatus, or small slivers of plastic from the packaging of the toner, as from the rough rim of a plastic toner bottle. Another source of contamination is lint or fibers which circulate through the machine as a result of internal air flow. In the case of scavengeless development, such foreign particles are likely to become ensnared in, or damage, the electrode wires in the nip between the donor roll and the photoreceptor, causing scratching of the photoreceptor, or the developed latent image thereon. Thus, to ensure copy quality, the toner must be free of both agglomerated clumps and foreign material, particularly in the case of scavengeless development.
U.S. Pat. No. 4,100,611 to Jugle discloses a developer apparatus for xerography, wherein a negative pressure is created in the developer housing to prevent migration of particles out of the housing. Defined inside the developer housing is a filter surface comprising a filter of predetermined pore size. A source of suction is provided on the opposite side of the filter surface.
U.S. Pat. No. 4,394,086 to Hoffman, Jr., et al. discloses a dirt barrier for use at the interface between the surface of a moving photoreceptor belt and the open section of a developer housing. The barrier is contoured to follow the surface of the photoreceptor belt and is spaced to minimize air flow between the barrier and the belt.
U.S. Pat. No. 4,878,090 to Lunde discloses an apparatus for developing a latent image with a liquid toner, that is a developer material in which toner particles are suspended in a liquid solution. The developer apparatus includes a housing, or shroud, to contain the liquid developer. The vacuum pump associated with the housing causes a current of air to circulate around the area of contact with the photoreceptor, in order to strip excess liquid developer from the photoreceptor.
U.S. patent application Ser. No. 07/758,993, assigned to the assignee of the present application, describes a development apparatus having an active airflow system for creating airflow out of a developer housing, drawing airborne contaminants therefrom. A developer system includes a plurality of developer housings movable into operative and inoperative positions, as needed. As each developer housing is moved into and out of its operative position, an air duct is rotated therewith for interfacing with or sealing off airflow through the housing, as appropriate.